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Gravity bends light, but can it also magnify itself?

Einstein made his fame with the idea that gravity bends light, a notion proven by observations of the planet Mercury in 1919. Once a crazy idea, it has shaped how scientists think about gravity ever since. Even now, astrophysicists aim telescopes at the violent regions around distant black holes, hoping the intense pull of those collapsed stars will fully reveal gravity's secrets. But up close in our solar system, a certifiably "crazy idea," according to National Science Foundation cosmologist Michael Turner, may offer a test of Einstein's theory of gravity.

Albert Einstein's theory of General Relativity may get tested by one cosmologist's "crazy idea."

AP

The idea comes courtesy of astronomer Robert Nemiroff of Michigan Technological University, who proposed it at last week's American Astronomical Society meeting in Washington, D.C. We know gravity bends light, but he proposes that "gravity might magnify itself," as well. If true, he suggests that regions of magnified gravity may exist in our solar system near the orbit of the planet Neptune. These "gravitational hollows" — or spikes of magnified gravitational pull — would exist at the point where the sun's gravity focuses the gravity from a distant star or black hole, much like a magnifying glass focuses light at a certain point. He compares how gravity looks around a star to the looks of a sea urchin, with hollows radiating away from the sun like sea urchin spines.

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For a look at an abstract on the topic of gravity lensing, follow this link from the American Astronomical Society meeting.

How do you test this idea? Fly a spacecraft though a region of magnified gravity and see if it slows down. The largest hollow would come from the giant star Sirius, whose gravity, if magnified, would create a hollow almost 40 miles wide. An atomic clock like the ones made now by the National Institute of Standards & Technology would be sensitive enough to pick up the difference in travel time, Nemiroff says. Much smaller hollows would be created by the pull of Polaris, the North Star, and the massive black hole at the center of the Milky Way galaxy.

Nemiroff says he has polled experts on Einstein's theory of gravity, better known as General Relativity, and they aren't certain whether gravity could bend gravity like it bends light. Turner is dubious, but adds "We know Einstein didn't have the last word on gravity. We're all looking for an idea crazy enough to be correct and this definitely is a crazy idea." Also cautious is astrophysicist Michael Martin Nieto of Los Alamos National Laboratory who says, "It is fun to think about, but for now Nemiroff's comments are intuitive," and not formally defined enough for other physicists to critique.

Still, Nemiroff says either way his idea merits investigation. If true, it would shake up our understanding of General Relativity and if false, it might tell us something about how gravity will perform around those distant black holes that cosmologists plan to investigate in coming decades.